1. Field of the Invention
The present invention relates to a system for driving an automotive vehicle and, more particularly, to a vehicle drive system so adapted as to control the driving of the vehicle with a motor as well as with an engine.
2. Description of the Related Art
There is recently the growing tendency that four-wheel drive cars are increasing, which have either of their front wheels and rear wheels driven with an internal combustion engine as primarily driven wheels and the other wheels driven with a motor as secondarily driven wheels, in order to decrease the weight of the vehicle body or for other reasons.
Japanese Patent Unexamined Publication No. 2-120,136 discloses a vehicle drive system which is so adapted as to change the driving force to be applied to the secondarily driven wheels from two motors having different driving forces. This vehicle drive system is such that the kinds and the number of the motors to be employed for driving the secondarily driven wheels are changed in accordance with the speed range of the transmission.
Further, Japanese Patent Unexamined Publication No. 57-74,222 discloses a vehicle drive system in which the function of a differential is added so as to have the hydraulic pressure supplied and distributed automatically to the left and right hydraulic motors (hydraulic cylinders) in accordance with the road surface load incurred upon the left and right secondarily driven wheels.
In addition, Japanese Patent Unexamined Publication No. 63-38,031 discloses a vehicle drive system employing left and right two electrically drivable motors which are so arranged as to make the torques generated by the motors constant by making the generated voltage larger as the vehicle speed becomes higher and which allows a manual switch to shift the status of the driving with the motors between execution and suspension of the driving.
The following should be taken into account, however, for the vehicle so arranged for its secondarily driven wheels as to be drivable with a motor.
First, it is of significance for such a vehicle to ensure stability in running straight and to set a preferred cornering characteristic; however, it is considerably difficult to meet such two requirements to a satisfactory extent. The vehicle characteristics comprising stability in running straight and the preferred cornering characteristic can basically be ensured by the setting of a suspension. In addition, such vehicle characteristics can be ensured by employing a unique device or system such as a rear wheel steering system or device.
Secondly, for such a vehicle as arranged for its secondarily driven wheels to be driven with the motors, it is of significance to determine a situation or condition in which the secondarily driven wheels are to be driven with the motor. It is not preferred in terms of fuel economy that the secondarily driven wheels be driven at random because the driving energy for driving the motor has to be gained eventually from the engine. Further, it is not preferred, too, that the execution of driving the motor is restricted more often than necessary from the point of view of saving energy, because the significance of driving the secondarily driven wheels with the motor may not become very meaningful. In addition, it can be noted that the four-wheel driven state, in which the secondarily driven wheels are driven with the motors, differ from the two-wheel driven state, in which the driving of the motor is suspended, to a great extent in terms of vehicle characteristics such as stability of the vehicle body and cornering characteristic.
As is apparent from the description as made hereinabove, it can be understood that, even if a driving region in which the secondarily driven wheels are to be driven with the motor is set as constant from the point of view of fuel economy, for example, if the driving region is set on the basis of the time of acceleration only, the problems as described hereinabove cannot be solved in a sufficient fashion.
Thirdly, when the left and right secondarily driven wheels are driven with the motors, it is advantageous in terms of effective transmission of the driving force to the road surface that the left and right secondarily driven wheels are discretely and separately driven with the left and right motors, respectively. However, if the road situation varies to a great extent between the road portion at which one of the secondarily driven wheels is located and the road portion at which the other is located, the driving of the left and right secondarily driven wheels discretely and separately may sometimes suffer from the disadvantage in terms of ensuring stability of the vehicle body, because the situation may become likely to occur such that one of the secondarily driven wheels can transmit its driving force effectively onto the road surface while the other is likely to cause slipping.
Fourthly, it should be noted that the driving of the secondarily driven wheels with a conventional hydraulic motor is made by converting a portion of the torque generated by the engine into a form of hydraulic pressure and directly utilizing such hydraulic pressure as it is, and that the vehicle is not driven with any torque other than the torque generated by the engine. In other words, the torque generated by the engine is divided and distributed simply for driving the primarily driven wheels and the secondarily driven wheels. This does not cause any problem if the vehicle with such arrangement is considered as a simplified four-wheel drive car for running snow road for a long period of time.
Recently, in order to enhance acceleration or for other reasons, it is considered to temporarily drive the secondarily driven wheels with a motor. In this case, even if the secondarily driven wheels are driven with the conventional hydraulic motor, the driving torque for driving the secondarily driven wheels having the torque larger than the torque generated by the engine cannot be provided. Hence, this arrangement gives rise to the situation in which requirements for improving acceleration or other matters cannot be met to a sufficient extent.